In an air-ingesting turbomachine (e.g., a gas turbine), air enters a compressor and is progressively pressurized as it is routed towards a combustor. The compressed air is premixed with a fuel and ignited within a reaction zone defined within the combustor, thus producing high temperature combustion gases. The combustion gases are then routed from the combustion chamber via a liner and/or a transition piece into a turbine section of the turbomachine where the combustion gases flow across alternating rows of stationary vanes and rotor blades which are secured to a rotor shaft. As the combustion gases flow across the rotor blades, kinetic and/or thermal energy are transferred to the rotor blades, thus causing the rotor shaft to rotate.
To increase turbine efficiency, modern combustors are operated at high temperatures which generate high thermal stresses on various mechanical components disposed within the combustor. As a result, at least a portion of the compressed air supplied to the combustor is used as cooling air to cool these components. For example, particular combustors include a generally annular combustor cap assembly that at least partially surrounds one or more fuel nozzles within the combustor. Certain combustor cap assembly designs include a cap plate that is disposed at a downstream end of the combustor cap assembly. The fuel nozzles extend at least partially through the cap plate which is typically disposed substantially adjacent to the combustion chamber. As a result, the cap plate is generally exposed to extremely high temperatures.
One way to cool the cap plate is to route a portion of the compressed air as cooling air into the combustor cap assembly and onto an upstream side of the cap plate. The cooling air is then routed through multiple cooling or effusion holes which extend through the cap plate. The cooling air flows from the effusion holes into the reaction zone defined within the combustor. This method is known in the industry as effusion cooling. However, the cooling air flowing through the multiple cooling holes enters the reaction zone unmixed with the fuel and at a temperature which is much lower than the combustion flame temperature. As a result, NOx and/or CO2 generation may be exacerbated and overall turbine efficiency may be decreased. In addition, the cooling capacity of the cooling air is not fully optimized, thus reducing the cooling efficiency of the combustor. Therefore, an improved system and method for utilizing cooling air within the combustor cap assembly would be useful.